1. Field of the Invention
The present invention relates to a surface acoustic wave device having a so-called face down structure comprising a base member and a piezoelectric substrate having a surface acoustic wave element formed on its one main surface and configured by opposing the one main surface of the piezoelectric substrate to an upper surface of the base member, and an electronic circuit device in which the surface acoustic wave device is disposed on a circuit board.
2. Description of the Related Art
In recent years, surface acoustic wave filters have been widely utilized in a communication field, and particularly are frequently used for mobile communication apparatuses. The mobile communication apparatus has evolved to a multifunctional apparatus also having functions serving as a digital camera and a radio receiver and further the function of remotely controlling home electric appliances.
In such flow, the number of devices employed for mobile communication apparatuses tends to be increased. However, the size of the mobile communication apparatus is difficult to increase from a request of the market. Therefore, it is required to reduce the size and the height of a device employed for the mobile communication apparatus.
Surface acoustic wave filters so far used for mobile communication apparatuses generally have a so-called CSP (Chip Size Package) structure mounted on ceramic substrates in a face down structure. In recent years, CSP structures using resin substrates have been proposed from reasons such as the reason that fabrication is easy.
An example of a surface acoustic wave device 1′ having a CSP structure using such a resin substrate is illustrated in FIG. 16. Here, “2” denotes a mounting base member composed of a resin substrate having a through hole 9 formed therein. A lead electrode 10 having a via conductor formed in the through hole 9 is provided on a lower surface of the mounting base member 2. A connecting pad 8 and a solder bump 7 are formed in an area, where the through hole 9 is positioned, on an upper surface of the mounting base member 2, and are connected to an excitation electrode 5 formed on a lower surface of a piezoelectric substrate 3 comprising a surface acoustic wave element and a signal electrode 6 electrically connected thereto.
The surface acoustic wave element is thus mounted on the mounting base member 2 in a face down bonding structure, thereby configuring the surface acoustic wave device 1′. In the drawings, “11” denotes an insulator (a solder resist film) for preventing, when the surface acoustic wave device 1′ is mounted on a circuit board, described later, through a solder, lead electrodes 10 from being short-circuited by the solder.
FIGS. 17 and 18 are cross-sectional views showing the steps of mounting the surface acoustic wave device 1′ on a circuit board 15 in an electronic circuit device.
A connecting pad 12 for connection is formed on an upper surface of the circuit board 15. A solder bump 13 is formed on the connecting pad 12.
When the surface acoustic wave device 1′ is mounted on the circuit board 15, portions, just below the signal electrode 6 and the connecting pad 8, of the lead electrode 10 on the lower surface of the mounting base member 2 must be positioned just above the solder bump 13, the positions where conductors on the side of the surface acoustic wave device 1′, i.e., the signal electrode 6 and the connecting pad 8 are formed are limited.
When an attempt to connect the surface acoustic wave device 1′ to the circuit board 15 in a state where a recess exists in the lead electrode 10 formed within the through hole 9 in the surface acoustic wave device 1′, as shown in FIG. 17, the thickness of the solder bump 13 cannot be accurately controlled in mounting the surface acoustic wave device 1′, so that a clearance W may easily occur on a lower surface of the insulator 11, as shown in FIG. 18. A solder in the solder bump 13 flows into the clearance W, so that the solder may be brought into contact with the other lead electrode 10 and the connecting pad 12 and short-circuited. If such short occurs, the excitation electrodes 5 are electrically connected to one another.
Furthermore, in the case of the connection by the solder bump 13, bubbles V may remain within the recess of the via conductor, as shown in FIG. 18. If water is included in the bubbles V, the water vaporizes by heat at the time of solder reflow and heat generated from the surface acoustic wave device itself at the time of application of a high power so that the bubbles V rapidly expand, which may cause some phenomena (hereinafter referred to as popcorn phenomena). For example, separation occurs between the mounting base member 2 and the piezoelectric substrate 3, and cracks appear in the lead electrode 10 and the solder bump 13 by a shock due to such rapid expansion of the bubbles V.
Consequently, the reliabilities of the surface acoustic wave device and the electronic circuit device using the same are reduced.
Furthermore, in a surface acoustic wave device comprising a plurality of surface acoustic wave elements having passbands of different frequencies, a device for preventing electromagnetic interference between the surface acoustic wave elements has been required.
An object of the present invention is to provide a versatile surface acoustic wave device having a large degree of freedom of the position where a conductor is formed and an electronic circuit device using the same.
Another object of the present invention is to provide a highly reliable surface acoustic wave device which prevents the flow of a solder to the utmost and has no problems such as inferior connection and an electronic device using the same.
Still another object of the present invention is to provide, in a surface acoustic wave device comprising a plurality of surface acoustic wave elements having passbands of different frequencies, a surface acoustic wave device being small in size and superior in isolation properties between the surface acoustic wave elements and reliability and an electronic circuit device using the same.